Micro-electromechanical systems (MEMS) have found a variety of optical applications. MEMS can include tiltable micromirrors for redirecting optical beams, movable optical fiber tips for optical switching, movable micromirrors for tuning a resonant wavelength of an optical cavity, and the like.
Tiltable optical grating micromechanical structures have been used in the prior art to tune laser emission wavelength. Referring to FIG. 1, a grating-tunable semiconductor laser 10 includes a laser chip 12, a lens 14, and a microfabricated diffraction grating 16 mounted on a pivot 11 over a pair of electrodes 18a , 18b. When a voltage is applied between the diffraction grating 16 and either of the electrodes 18a or 18b, the diffraction grating 16 attracts to the respective electrodes 18a or 18b, tilting by a controllable angle about an axis defined by the pivot 11. The reflection wavelength of the diffraction grating 16 and, consequently, the emission wavelength of the laser 10 is tuned by applying voltage between one of the electrodes 18a or 18b and the diffraction grating 16. Tunable semiconductor lasers of this type have been described by Masaya et al. in Japanese patent 3129890A; Hiroyasu in Japanese patent application publication 2007-234916; Liu et al. in an article entitled “Tunable laser using micromachined grating with continuous wavelength tuning”, Appl. Phys. Lett. 85, No. 17, pp. 3684-3686; Syms et al. in an article entitled “MOEMS Tuning Element for a Littrow External Cavity Laser”, J. of MEMS v. 12, No. 6, p. 921-928; and other authors.
To improve diffraction efficiency, diffraction gratings of the prior art MEMS devices were “blazed”, that is, grating grooves 17 were shaped to be approximately perpendicular to incoming optical beams 15. Liu and. Sims blazed their gratings by Deep Reactive Ion Etching (DRIE) the required triangular profile of the grooves 17 into a side 19 of the diffraction grating 16. Due to a limitation on the maximum etching depth, only limited height gratings can be produced using a DRIE side etching technique. Furthermore, optical quality of side-etched grating surfaces is usually not as good as optical quality of surfaces disposed parallel to the substrate. Lower optical quality reduces achievable diffraction efficiency in blazed gratings manufactured by side etching.
Notably, high diffraction efficiencies can be achieved in binary diffraction gratings in “−1st” diffractive order, even though the surfaces of the grooves and ridges of the grating may not be perpendicular to the incoming optical beam. By way of example, A. Hessel et al. in an article entitled “Bragg-angle blazing of diffraction gratings ”JOSA Vol. 65 No 4 April 1975, p. 380-384, discussed binary gratings with high diffraction efficiency. Kiang et al. in an article entitled “Surface micromachined diffraction gratings for scanning spectroscopic applications”, Proc. Int. Conf Sol-State Sensors and Actuators, June 1997, disclose a tunable binary diffraction grating manufactured in polycrystalline silicon using reactive ion etching (RIE) of a silicon dioxide layer; depositing polycrystalline silicon in the trenches in the silicon dioxide; and releasing the polycrystalline silicon grating structures. The grating of Kiang et al. is assembled into operating position of about 45 degrees to the substrate, and is coated with a layer of reflective metal. Detrimentally, assembly of out-of-plane tunable diffraction gratings of Kiang et al. requires many sophisticated process steps. Self-assembly of the diffraction grating into an out-of-plane position requires that the metal layer is deposited after the grating assembly step. Electrical shorts can occur when the metal deposition step occurs after the assembly step. Therefore, the metal layer cannot be made thick, which may limit maximum achievable reflectivity and/or diffraction efficiency. Furthermore, large out-of-plane standing structures supported by relatively thin and flexible hinges are sensitive to shock and vibration.
Grating reflection wavelength can also be tuned by providing a stretchable optical grating structure. In stretchable gratings, the reflection wavelength is varied when the groove spacing changes upon stretching. By way of example, Stanley et al. in U.S. Pat. No. 7,826,501; and Tormen et al. in an article entitled “Deformable MEMS grating for wide tunability and high operating speed”, J. Opt. A: Pure Appl. Opt. 8 (2006) S337-S340, disclose microfabricated blazed diffraction gratings having an array of parallel elongated reflective surfaces disposed on a stretchable support. When the support is stretched, the groove spacing is changed, effectively tuning the reflected wavelength. Detrimentally, stretchable diffraction gratings are not as reliable, and the optical quality of stretchable gratings is generally lower than the optical quality of solid gratings.
Another type of wavelength tunable diffractive structures includes a phased array of individually tiltable or movable mirrors. For example, Belikov et al. in U.S. Pat. No. 7,042,920 disclose a tunable laser having a phased array grating including individually tiltable or translatable micromirrors allowing one to vary pitch and/or effective angle of the diffractive structure. Detrimentally, the phased array of Belikov et al. requires a complex controller/driver, and is rather difficult to manufacture.
The prior art is lacking an easily tunable, highly manufacturable and reliable diffractive MEMS device capable of attaining high optical efficiency over the tuning range. Accordingly, it is a goal of the invention to provide such a tunable diffractive MEMS device.